Storage of humic fertilizers from leonardite and changes in their chemical composition
Until recently, when using leonardite for fertilization, its preparation and composting or mixing were carried out exclusively by consumers. However, such production forms could not fully meet the growing needs of agriculture, which necessitated the organization of industrial production of humic fertilizers.
Currently, the agricultural market faces the task of mass production of organo-mineral humic fertilizers, which requires solving a number of organizational issues, primarily the timing of fertilizer production and the duration of their storage.
The objective of our research was to study changes in the chemical composition of humic fertilizers during their storage over a long period.
Experiments were conducted in the Chernihiv and Zhytomyr regions at different enterprises.
Samples for research were collected with a probe along the ridge of stacks perpendicular to the plane of the bottom and along the slope perpendicular to its plane every half meter by horizons. To collect one sample, three drillings were made at one location but at three points, gradually deepening each point. Thus, samples from three probe scoops collected at the same depth constituted an average sample. Sampling was carried out at specific storage intervals.
Dynamics of temperature, ash content, and moisture
Observations of temperature, moisture, and ash content were conducted in the Zamhlai district of the Chernihiv region. After 20 days of storage, the temperature throughout its profile corresponded to the ambient temperature. After 70 days of storage during the summer period, a sharp temperature increase to 50-57°C was recorded, which remained within these limits for 35 days, after which it began to decrease.
Temperature changes were also observed along the profile of the stack: it reached its maximum at a depth of 1-2 m and then gradually decreased. Further observations of temperature in fertilizer stacks showed that even at the same enterprise, heating does not occur simultaneously, which apparently depends on the moisture and ash content of the raw material.
It should be noted that the relatively high moisture content of leonardite (50-60%), which is a necessary condition for fertilizer production, slows down the temperature increase in the stack to higher limits, thereby maintaining optimal conditions for microorganism activity.
Ash content remained almost unchanged over storage periods. The observed fluctuations are more likely due to uneven ash distribution in the original product than to losses of organic matter.
Dynamics of total nitrogen and its forms
The content of total nitrogen, as shown by observations in the Zamhlai district, is not stable during storage; it changes. Apparently, during storage, there is a redistribution of total nitrogen reserves. By the time the experimental stacks were completed, the content of ammonium nitrogen in leonardite + NPK significantly predominated over nitrate nitrogen. During storage, an increase in mobile forms of nitrogen was observed. The maximum amount was recorded in the fourth and sixth months, during which the content of ammonium nitrogen predominated over nitrate nitrogen. By the one-year storage mark, the amount of mobile forms of nitrogen in leonardite + NPK had significantly decreased, but it was still higher than the initial amount. By this time, ammonium nitrogen had almost completely converted to nitrate nitrogen.
As an example, Table 1 presents some data from one of the Chernihiv enterprises.
| Storage period, months | Forms of nitrogen | Content | ||
|---|---|---|---|---|
| 3 month | 6 month | 12 month | ||
| Content at one-meter depth | Hydrolyzable | 0.344 | 0.354 | 0.292 |
| including ammonium | 0.242 | 0.125 | 0.092 | |
| nitrate | 0.102 | 0.229 | 0.200 | |
| Content at two-meter depth | Hydrolyzable | 0.346 | 0.497 | 0.307 |
| including ammonium | 0.159 | 0.063 | 0.071 | |
| nitrate | 0.187 | 0.434 | 0.236 | |
| Average for the stack | Hydrolyzable | 0.330 | 0.388 | 0.341 |
| including ammonium | 0.212 | 0.175 | 0.094 | |
| nitrate | 0.118 | 0.213 | 0.246 | |
The content of mobile forms of nitrogen increases with sampling depth up to two meters, after which a gradual decrease is observed.
At different enterprises, the accumulation of mobile forms of nitrogen varies in experimental stacks by month. For example, at an enterprise in the Zhytomyr region, the maximum amount of mobile nitrogen in the experimental stack was recorded at the beginning of the third month after storage began; in the Zamhlai district, it was in the second decade of the fifth month; and at a third enterprise, only in the seventh month of fertilizer storage. Without experimental data for an objective assessment of this phenomenon, we can only say that the storage conditions of the stacks are not identical. Apparently, the ability of leonardite to self-heat plays a decisive role in this.
The redistribution of nitrogen in the stack and changes in its forms during storage should likely be explained by microbiological processes occurring in the mass of humic fertilizers, as all necessary conditions are created for this. Possibly, in the stack, enzymatic breakdown of organic substances in leonardite occurs with the release of ammonia. This ammonia, remaining in a diffuse state, moves along with water vapor to the upper boundaries of the stack. Some of the ammonia may nitrify. Nitric acid, in turn, reacting with ammonia, forms ammonium nitrate.
While explaining the movement of nitrogen in the stack of humic fertilizers during storage and emphasizing the importance of microbiological processes, we cannot ignore the idea that the temperature regime also influenced these transformations. However, considering that the temperature in the fertilizer stack is lower than in the leonardite stack, we can assume that the role of the thermal factor itself is less significant in the storage of humic fertilizers than in the storage of leonardite.
Changes in phosphorus mobility
Conducted analyses showed (Table 2) that the amount of phosphorus compounds soluble in 0.5 N H₂SO₄ in fertilizers increases during storage. It also changes along the stack profile.
| Name of deposits | Storage period, months | |||||
|---|---|---|---|---|---|---|
| 2 | 3 | 4 | 5 | 6 | 11 | |
| Zamhlai district (Chernihiv region) | 0.192 | - | 0.313 | 0.337 | - | 0.421 |
| Velyke (Kyiv region) | 0.394 | - | 0.391 | - | - | 0.703 |
| Note: "-" — not determined. | ||||||
Up to the 140-day storage period, maximum solubility was observed at a depth of 1.5-2.5 m, after which the amount of mobile forms of phosphoric acid significantly increased and became more evenly distributed throughout the stack profile, reaching a maximum on the 340th day. As for water-soluble phosphorus, a similar trend toward increased solubility by the end of storage was observed.
The general pattern—an increase in mobile forms of phosphorus during fertilizer storage—should likely be explained by the mobilization of phosphorus from hardly soluble phosphates in the presence of formed nitric acid. The possibility of forming mobile phosphorus-organic compounds cannot be excluded, as it is known that phosphoric acid can form complex compounds with humic acid. However, experimental data on this issue have not yet been obtained.
Mobility of humic acids and changes in pH
As analyses showed, during the storage of humic fertilizers in the experimental stack in the Zamhlai district, the pH of the aqueous extract shifted toward alkalinity up to the 90-120-day mark, after which, with increasing storage duration, the alkalinity decreased and sometimes reached acidity. Changes in reaction were also observed along the stack profile. Maximum alkalinity was achieved at a depth of 2-2.5 m, after which alkalinity began to decline, with acidic reactions observed in the surface layers of the stack.
It is interesting to note that the increase in pH paralleled the accumulation of ammonium nitrogen, while the decrease corresponded to an increase in nitrate nitrogen. The described phenomena were also observed in experimental stacks in Velyke, Kyiv region.
During the storage of humic fertilizers, the amount of water-soluble forms of humic acids increased up to a certain period. Maximum accumulation of water-mobile forms of humic acids occurred in the fifth-sixth month of storage. During this period, leonardite + NPK also accumulated the maximum amount of mobile forms of nitrogen. The content of humic acids also changed along the stack profile. If we compare the graphs characterizing the amount of water-soluble humic acids and the amount of ammonium nitrogen, a parallel is easily noticeable, meaning that ammonia released during the decomposition of organic matter in leonardite is captured by humic acids, forming soluble salts—ammonium humates. Later, as storage duration increased, the content of water-soluble humic acids decreased, especially in the surface layers of the stack. It should be noted that the decrease in solubility of humic acid in these layers almost paralleled the accumulation of nitrate nitrogen. Thus, nitric acid causes the coagulation of humic acids, so the accumulation of this form of nitrogen is undesirable from this perspective.
Summarizing the above, we can say that during the one-year storage of humic fertilizers, there is an absolutely insignificant decrease in the content of water-soluble humic acids and acidification of the fertilizer itself. Ammonium nitrogen almost completely converts to nitrate nitrogen during this period. Such fertilizers can no longer be called peat-mineral-ammonium-humic, as they do not contain water-mobile humic acids and ammonium nitrogen during this period. Apparently, it is better to call them organo-mineral-nitrate during this period.
In cases where there is a decrease in the content of water-soluble forms of humic acids, additional treatment of such fertilizers with ammonia water can be recommended, calculating its dose based on the free absorption capacity. The accumulated nitric acid will be neutralized by ammonia, fertilizers with an acidic reaction will become slightly alkaline, and humic acids will return to a water-soluble state.
In conclusion, it should be noted that the optimal stack height for storing humic fertilizers is 3.5-4 m. Stacks taller than this are impractical, as nutrient accumulation in deep layers is insignificant.
Results of microvegetation and field experiments
To study the fertilizing qualities of stored humic fertilizers, microvegetation and field experiments were conducted with tomatoes, cabbage, corn, potatoes, sugar beets, and flax.
In laboratory conditions, a microvegetation experiment was set up in sand culture with tomatoes to determine changes in the fertilizing qualities of humic fertilizers depending on their position in the stack during storage. The weight of sand in the vessel was 1 kg, the number of plants was 15, with four replications. For the experiment, samples of leonardite + NPK from the Kyiv region after six months of storage were taken from the stack at depths of 0.5, 1.5, 2, and 3 m. Fertilizer doses of 20 g of absolutely dry substance per vessel were applied instead of nitrogen and phosphorus in the Pryanishnikov mixture. The experiment began on February 15, 2020, and ended after three weeks. The absolutely dry weight of sprouts and the uptake of nitrogen and phosphorus per vessel were recorded (Table 3).
| Experiment scheme, indicators | Complete Pryanishnikov mixture (control) | leonardite + NPK, sampled at depth, m | |||
|---|---|---|---|---|---|
| 0.5 | 1.5 | 2 | 3 | ||
| Green weight of 10 plants, mg | 3550 | 3920 | 5250 | 5130 | 5610 |
| Absolutely dry weight of 10 plants, mg | 208 | 304 | 326 | 438 | 417 |
| Mobile nitrogen applied per vessel, mg | 77.5 | 80.0 | 77.0 | 80.0 | 98.0 |
| Nitrogen uptake per vessel, mg | 8.6 | 7.3 | 8.8 | 12.4 | 12.7 |
| Nitrogen utilization coefficient | 11.09 | 9.12 | 11.42 | 17.47 | 12.96 |
| Mobile P₂O₅ applied per vessel, mg | 82.0 | 48.0 | 58.0 | 46.0 | 50.0 |
| P₂O₅ uptake per vessel, mg | 4.80 | 4.80 | 4.00 | 4.80 | 5.93 |
| P₂O₅ utilization coefficient | 5.85 | 10.0 | 6.70 | 10.43 | 11.87 |
The results of the experiment clearly show that the utilization coefficient of various nutrients in leonardite + NPK is higher than in the Pryanishnikov mixture, and it increases with sampling depth, meaning that leonardite + NPK did not lose its fertilizing qualities after six months of storage.
On March 30, 2020, a second microvegetation experiment was set up in sand culture with tomatoes. The objective was to compare the fertilizing qualities of humic fertilizers obtained from different peat enterprises after seven months of storage. For this purpose, identical samples were taken from stacks in the Zamhlai district of the Chernihiv region, as well as from enterprises in the Zhytomyr and Kyiv regions. Doses of leonardite + NPK of 10 g per vessel in terms of absolutely dry weight were applied instead of mineral nitrogen and phosphorus in the Pryanishnikov mixture. The experiment ended on April 24. Table 4 presents the data from this experiment.
| Experiment scheme, indicators | Complete Pryanishnikov mixture (control) | Names of enterprise locations | |||
|---|---|---|---|---|---|
| Zamhlai | Kiselev | Buchman | Kodra | ||
| Green weight of 100 plants, g | 58.467 | 48.797 | 56.205 | 60.00 | 62.115 |
| Absolutely dry weight of 100 plants, g | 5.029 | 4.489 | 3.721 | 4.454 | 4.550 |
| Mobile nitrogen applied per vessel, mg | 77.5 | 39.0 | 56.0 | 28.6 | 58.4 |
| Nitrogen uptake per vessel, mg | 23.2 | 22.2 | 15.0 | 21.7 | 23.5 |
| Nitrogen utilization coefficient | 29.9 | 56.9 | 26.7 | 75.8 | 40.2 |
| Mobile P₂O₅ applied per vessel, mg | 82.0 | 24.6 | 14.4 | 19.2 | 29.0 |
| P₂O₅ uptake per vessel, mg | 4.1 | 3.5 | 5.4 | 3.3 | 4.9 |
| P₂O₅ utilization coefficient | 5.0 | 14.2 | 37.5 | 17.1 | 16.9 |
| Note: The table shows the results of the experiment on samples taken from stacks at a depth of 1 m along the slope. | |||||
In this experiment, the utilization coefficient of nitrogen and phosphorus by plants fertilized with leonardite + NPK was generally significantly higher than with the Pryanishnikov mixture. It should be noted that experimental plants fertilized with humic fertilizers in microvegetation experiments were more developed than control plants in all cases. This suggests that plants utilize not only mobile nitrogen and phosphorus but also their more stable forms.
This idea and all the above points were confirmed in the results of field experiments comparing the effectiveness of freshly prepared leonardite + NPK and after winter storage. As an example, we present the results of experiments (Table 5) conducted in the Repkyn district of the Chernihiv region in 2020. The experiments were set up in three replications on dark gray podzolized soils.
| Experiment scheme | Corn | Sugar beet | Flax | |||||
|---|---|---|---|---|---|---|---|---|
| Total green mass, c/ha | Including cobs | Increase c/ha | Compared to control, % | Yield, c/ha | Increase c/ha | Yield | Increase | |
| Control (no fertilizers) | 146.3 | 64.8 | - | - | 343.6 | - | 493.0 | - |
| Adept, 2 l/ha (3 times) | 275.0 | 141.7 | 128.9/76.9 | > 87.7 | 463.6 | 120.9 | 536.0 | 43.0 |
| leonardite + NPK from last year, 10 t/ha | 350.3 | 158.8 | 204.0/90.0 | > 138.9 | 459.0 | 115.6 | 700.0 | 207.0 |
| leonardite + NPK in quantity equivalent to 2 l/ha Adept, (3 times) | 256.3 | 133.7 | 110.0/68.9 | > 74.8 | 424.6 | 81.0 | 583.0 | 90.0 |
| Note: The numerator shows the yield increase of cobs, the denominator—green mass. | ||||||||
The table shows that the equivalent set of mineral fertilizer mixtures is inferior to leonardite + NPK, especially after winter storage.
In 2023-2024, leonardite + NPK from the Buchman enterprise, which had overwintered in a stack, was tested on fruit-bearing vineyards in the Zakarpattia region in two different micro-natural-climatic areas. Table 6 presents the results of fertilizer experiments on the experimental vineyard field.
| Experiment scheme | 2023 | 2024 |
|---|---|---|
| Control | Average yield, c/ha: 85.98 Increase, %: - |
Average yield, c/ha: 18.4 Increase, %: - |
| leonardite + NPK, 3 kg per bush | Average yield, c/ha: 112.00 Increase, %: 26.02 |
Average yield, c/ha: 19.9 Increase, %: 3.0 |
| NPK (equivalent to 3 kg leonardite + NPK) | Average yield, c/ha: 105.60 Increase, %: 19.62 |
Average yield, c/ha: 18.8 Increase, %: 2.2 |
Additionally, it should be noted that sugar content in berries increased in fertilized variants. The effect of humic fertilizers was particularly noticeable in this regard.
Conclusions
- During autumn-winter storage, changes occur in the chemical composition of humic fertilizers (leonardite + NPK), specifically:
- a) As a result of the decomposition of organic substances in leonardite, the amount of easily mobile forms of nitrogen increases. Maximum nitrogen mobilization in the stack is observed at a depth of 1.5-2.5 m;
- b) Hardly soluble phosphorus compounds transition to a more mobile state;
- c) The dynamics of pH and mobility of humic acids generally parallel changes in the content of ammonium and nitrate nitrogen. An increase in ammonium nitrogen in humic fertilizers is accompanied by an increase in pH and solubility of humic acids, while an increase in nitrate nitrogen leads to a decrease in pH and solubility of humic acids.
- Vegetation experiments established the high availability of nutrients in humic fertilizers stored for a long period.
- The yield of agricultural crops is higher when applying humic fertilizers after autumn-winter storage compared to fertilizing with leonardite + NPK and an equivalent set of mineral fertilizers.
- Storage of humic fertilizers during the autumn-winter period does not reduce their fertilizing qualities, but the optimal storage period varies depending on the properties of leonardite and in most cases ranges from 4-6 months. Stack size affects fertilizer quality. The optimal stack height for storage is 3.5-4 m.